Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: Templeton on 10/05/2008 21:24:08
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Templeton asked the Naked Scientists:
Dear Naked Scientists.
Last week you said that that nitrogen doesn’t dissolve. I thought Guinness had nitrogen bubbles?
T.
What do you think?
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When you open a can of beer or lemonade, you reduce the pressure, so dissolved gasses will come out of solution forming bubbles.
In fact nitrogen is added because it is less soluble. I making a good stable guiness head you want very small bubbles. The problem is that very small bubbles, particularly of carbon-dioxide are not stable. Surface tension acts to shrink the bubble creating a higher pressure in the bubble than outside it. This pressure difference causes the gas inside to dissolve in the beer. The smaller the bubble, the higher the pressure, so small bubbles tend to collapse as the gas inside is dissolved again.
To make very small bubbles stable you need to use a gas which doesn't dissolve very well - carbon-dioxide is very soluble so they use nitrogen instead. Once the bubble has grown to a critical size with nitrogen the pressure will reduce enough for the carbon-dioxide to come out of solution and the bubble will grow a bit more.
The widget in the bottom of the can is designed to squirt gas and beer out as you open it, so agitating the beer and forming more bubbles (a bit like shaking up the can)
I am not sure if they use something similar on pumps in pubs - it is probable you don't have to, as the beer in a pub is still alive, and so will have very different properties.
I expect they use nitrogen rather than
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See also this thread:
http://www.thenakedscientists.com/forum/index.php?topic=14262.msg172239#msg172239
And Google Nitrokeg...